Although cellular receptors for many human viruses have been identified in the past 10 years, we do not yet fully understand how binding of a virus to its receptor leads to penetration of the viral genome into the cell to initiate replication. We identified human aminopeptidase N (hAPN), a membrane metalloprotease, as the receptor for human coronavirus 229E. APN is the only protease shown to have virus receptor activity. The 229E viral envelope glycoprotein that binds to hAPN is the 200 kDa spike protein, S. Our goal is to analyze the molecular interactions between S and hAPN that lead to virus infection. The study of this is a novel virus-receptor system will provide new insight into how enveloped viruses bind to and fuse with cellular membranes. Human coronaviruses cause 15 to 30 percent of upper respiratory tract and sinus infections in humans of all ages, and lower respiratory tract infections and exacerbations of asthma in children. No vaccines or drugs are available to treat or prevent diseases caused by human coronaviruses. We will do structural and functional analyses of the 229E S glycoprotein and hAPN. We will introduce mutations into the S gene and the hAPN gene, express the mutant proteins, and explore the effects of the mutations on virus-receptor interactions. We will expressed anchorless, soluble S and hAPN glycoproteins in baculovirus vectors and purify the proteins to homogeneity. The structures of these glycoproteins, or peptides derived from them, will be analyzed by X-ray crystallography. We will determine whether binding of the purified receptor at neutral or acid pH, or acid pH alone can lead to conformational changes in the S protein on virions that may be associated with membrane fusion. We will select and/or engineer 229E viruses and VSV pseudotypes containing mutant S proteins and characterize the functional and antigenic changes that result from the mutations. The S gene will be sequenced by RT-PCR from human clinical specimens, and S proteins that differ significantly in amino acid sequence will be cloned, expressed in eukaryotic cells and their interactions with the hAPN receptor will compared with the wild type 229E S protein. In addition to providing a novel model system for studying virus-receptor interactions, our research on 229E S glycoprotein and hAPN may lead to development of new anti-viral drugs that block the initial stages of human coronavirus infection.